The present invention relates, in general, to a method of locating radiation emitters, and more specifically, to a method of determining azimuth and elevation angles of radiation signals from the emitter using a single-axis direction finding system.
Airborne platforms such as airplanes and helicopters have been used for detecting the location/geolocation of emitters (e.g., a radiation emission source such as a radar transmitter). Such platforms are often equipped with Direction Finding (DF) systems that measure the angle-of-arrival (AOA) of radiation originating from the emitter.
In order to fully define a true line of bearing to the emitter, the AOA is desirably determined using two orthogonally oriented measurement devices. These orthogonally oriented measurement devices are typically two distinct AOA measurement systems that measure respective orthogonal angles associated with the airborne platform to emitter direction with respect to the platform""s frame of reference.
The two orthogonal angles associated with the airborne platform to emitter direction are known as the azimuth angle and the elevation angle. The azimuth angle (az) is a horizontally measured angle associated with the direction of the airborne platform to the emitter. The azimuth angle is customarily measured with reference to magnetic north. Typically, the azimuth angle is greater than zero (az greater than 0) when measured from north to east, and the azimuth angle is less than zero (az less than 0) when measured from north to west (i.e., xe2x88x92180xc2x0xe2x89xa6azxe2x89xa6+180xc2x0). The elevation angle (el) is a vertically measured angle associated with the emitter to airborne platform direction. The elevation angle is customarily measured from a horizontal plane.
After the azimuth and elevation angles of arrival (az and el AOAs) are measured, coordinate transformations are performed to convert the AOAs to an earth-referenced coordinate system so that the data can be used by geolocation algorithms and by other, off-board users. In order to complete the coordinate transformation, the measurement aircraft""s 3-dimensional location and its angular orientation must be known. This data is usually available to a high degree of accuracy from the on-board GPS/INS (Global Positioning System/Inertial Navigation System) equipment.
This conventional method of measuring the azimuth and elevation AOAs suffers from various deficiencies. For example, the cost of a two axis system may be significant. Further, the two axis system may strain the antenna mounting and radar cross section limitations. As such, it is not always possible or desirable to install a full two-axes AOA measurement system on an aircraft.
In these cases, a single-axis AOA system may be employed. Single-axis systems are usually oriented to measure the emitter AOA with respect to the host aircraft""s azimuth frame of reference. Typically, the elevation AOA is either assumed to be zero, or it is estimated based on a number of factors including the measurement aircraft""s altitude. The estimate of the relative elevation angle is often somewhat inaccurate; this is particularly true when the aircraft is flying at high altitudes or when the emitter is located in an area that has significant terrain variations. If the elevation estimate is incorrect, the coordinate transformation can result in significant emitter azimuth AOA errors.
As such, it would be desirable to have an improved method of measuring the azimuth and elevation AOAs for an emitter.
In an exemplary embodiment of the present invention, a method of determining an azimuth and an elevation angle to a radiation emission source using a single-axis direction finding system is provided. The method includes receiving a plurality of radiation signals at the single-axis direction finding system. The plurality of radiation signals are emitted from the subject emitter, each of the plurality of radiation signals being received at one of a plurality of attitudes of the single-axis direction finding system. The method also includes measuring an angle of arrival of each of the plurality of radiation signals with respect to the single-axis direction finding system. Additionally, the method includes calculating an azimuth angle of each of the plurality of radiation signals with respect to the single-axis direction finding system using the respective measured angle of arrival. Further, the method includes calculating a respective vector corresponding to each of the azimuth angles at different candidate elevation angles within a predetermined range. Further still, the method includes determining the elevation angle of the radiation emission source with respect to the single axis direction finding system by determining the intersection of the vectors.
In another exemplary embodiment of the present invention, a method of determining an azimuth and an elevation angle to a radiation emission source using a single-axis direction finding system is again provided. The method includes receiving a plurality of radiation signals at the single-axis direction finding system. The plurality of radiation signals are emitted from the subject emitter, each of the plurality of radiation signals being received at one of a plurality of attitudes of the single-axis direction finding system. The method also includes measuring an angle of arrival of each of the plurality of radiation signals with respect to the single-axis direction finding system. Additionally, the method includes calculating an azimuth angle of each of the plurality of radiation signals with respect to the single-axis direction finding system using the respective measured angle of arrival. Further, the method includes calculating a rate of change of the azimuth angle with respect to the plurality of radiation signals. Further still, the method includes determining the elevation angle of the radiation emission source with respect to the single axis direction finding system using the calculated rate of change of the azimuth angle.